A. Field
The invention concerns an automatic firearm.
B. Related Art
For small calibre automatic firearms, there are two types of mechanisms according to the designated use, in particular:                Firing mechanisms from the closed bolt characterized by the presence of a cartridge in the ammunition chamber before firing (set off by applying pressure on the trigger), the operating group being in a ready to fire position and only pressure applied to the trigger releases the firing mechanism and strikes the ammunition; and,        Open bolt mechanism, characterized by the absence of a cartridge in the chamber before the start of firing (also set off by applying pressure on the trigger), the operating group being held in a back position and released when pressure is applied to the trigger, the mechanism must then insert a cartridge into the chamber and lock it before releasing the firing mechanism and striking the ammunition.        
The design for firing with closed bolt is used mainly for rifles because it makes the firearm more accurate, with a high hit probability with the first shot, and less susceptible to contamination from the environment (sand and dust, mud).
This design has the major drawback of allowing firing a limited number of cartridges over a given period (firing conditions), because after the number of cartridges have been shot, the chamber and barrel heat up considerably, and if the firing stops with a cartridge loaded in the chamber, it can heat up to the point of spontaneous combustion.
This self-ignition of the propellant in the cartridge is commonly called cook-off.
To counter this major drawback, an open bolt firing design is currently used. It is characterized by the fact that before firing, all the operating group is held in retracted position and that there is no ammunition in the chamber.
When pressure is applied to the trigger, the operating group is released: a cartridge is fed into the chamber, which is then struck at the end of the forward run of the mechanism.
This open bolt functioning principle is regularly used in machine-guns, providing them with a capability of firing high volumes of ammunition without the risk of self-ignition of the ammunition.
The inconvenience of this design is in the limited accuracy, which is not as good as it is with a firearm firing from the closed bolt position, and a lower resistance to fouling since when the firing stops, the mechanism is in the back position and the chamber is open and exposed to contamination from detritus, dust or other environmental pollution.
Some firearms have been designed to make up for these flaws. It is mainly the case of the FG-42 automatic rifle and the Johnson Machine Gun which were firing semi-automatic from the closed bolt position, and automatic firing from the open bolt position. These firearms only provide incomplete solutions to the problem, since the self-ignition risk still exists in semi-automatic firing mode and these firearms are still less precise and exposed to environmental pollution when they are used in automatic firing mode.